So Robin Cook has become the first casualty of the war.
The Commons leader has resigned on a matter of principle - the first New Labour minister ever to go on those grounds - and others are certain to follow.
To state the obvious, this is seriously bad news for Tony Blair.
The MP will become far and away the most serious and senior dissenter on the backbenches - and his attacks will not be confined to this war.
Once this is all over he has the potential to be a real and powerful threat to Tony Blair, sniping at any number of policies from Lords reform to foreign policy.
Indeed, his influence may prove to be less important over the war crisis than it will be later when politics returns to what will then count as normality.
Losing Mr Cook from the cabinet under ordinary circumstances would have troubled the prime minister little.
The former foreign secretary had been demoted by Mr Blair once and was always seen as a potential trouble maker.
It is quite possible that Mr Blair would have got around to reshuffling him out of government at some point.
He was not seen as a serious leadership challenger or even any longer as a focal point for Old Labour and left wing dissenters looking for a figurehead.
He has never been a great gang leader and previously found it difficult to attract followers most of who were attracted more to his rival Gordon Brown.
Put bluntly, he was seen as a spent force. That is no longer the case.
Ever since he was demoted he has used his position to build a base amongst backbenchers and make his mark through modernising parliament.
That has only been partially successful and the prime minister has stymied him on at least two occasions - once over changes to the appointment of select committee chairmen and again over reform to the House of Lords.
So Mr Cook has plenty of reason to feel less than superglue loyalty to his leader.
That is not, however, to suggest his action has been sparked by disloyalty. He clearly has reached his red line and is not prepared to step over it.
This is as much a matter of principle for him as it is for the prime minister.
And Mr Cook now has the real power to become the anti-war rebels' champion on the backbenches.
Possibly he remains vain enough to still see himself as a contender if this all goes wrong for Tony Blair.
More likely, he sees his role as Kingmaker in the event of that challenge. And in any case it is maintaining his integrity rather than other questions that are in the front of his mind at the moment.
However, Mr Cook is a supremely accomplished and astute political operator. So he has probably calculated the odds are heavily against a leadership challenge.
In that case, he has given up a job that was already running out of steam - thanks to his leader - on a matter of principle.
His biographers will give him a good write up for that.